Coenzyme A (CoA) is a cofactor of ubiquitous occurrence in plants, bacteria, and animals needed in a large number of enzymatic reactions central to intermediary metabolism, including the oxidation of fatty acids, carbohydrates, and amino acids. Coenzyme A is the common acyl carrier in prokaryotic and eukaryotic cells required for a multitude of reactions for both biosynthetic and degradative pathways amongst others forming derivatives that are key intermediates in energy metabolism [Rubio06].

Pantothenate (vitamin B5) is the universal precursor for the synthesis of the 4'-phosphopantetheine moiety of CoA and of acyl carrier proteins. Only plants and microorganisms can synthesize pantothenate de novo. In contrast to the bacterial biosynthesis of pantothenate (phosphopantothenate biosynthesis I), plant pathways for pantothenate formation are incomplete.

Coenzyme A (CoA), derived from pantothenate is a cofactor of ubiquitous occurrence in plants, bacteria and animals needed in a large number of enzymatic reactions central to intermediary metabolism, including the oxidation of fatty acids, carbohydrates, and amino acids.

About This Pathway

Feeding studies in Pisum sativum leaf discs support the existence of a similar biosynthesis pathway of pantothenate via pantoate: when fed with [14C]valine, the radiolabel was incorporated into α-oxoisovalerate, oxopantoate and pantoate [Jones94b]. The last enzyme of the pathway: pantothenate synthetase catalyzes the formation of pantothenate from pantoate and β-alanine. The origin of plant β-alanine is uncertain but appears to be different from that of bacteria which synthesize it from L-aspartate. Instead plants seem to produce β-alanine from a variety of other sources, like polyamines, uracil and propionate (see β-alanine biosynthesis II).

The biosynthesis of CoA is carried out from pantothenate in five enzymatic steps. All of the enzymes involved have been recently identified in Arabidopsis thaliana and the metabolic sequence confirmed in vitro by combining the recombinant enzymes [Kupke03].

Pantothenate kinase, the enzyme catalyzing the first step, phosphorylates 4'-phosphopantothenate to 4'-phosphopantothenate followed by the addition of cysteine resulting in the formation of (R)-4'-phospho-N-pantothenoylcysteine (PPC). PPC is decarboxylated in the next reaction to 4'-phosphopantetheine and then in two successive reactions converted to CoA by the enzymatic action of 4'-phosphopantetheine adenylyltransferase and dephospho-CoA kinase [Kupke03] [Steinbacher03a]. The reactions in the biosynthetic route towards CoA parallel the reaction sequence obtained in bacteria but differ regarding the functionality of the involved enzymes. In plants every step is catalyzed by single monofunctional enzymes, whereas in bacteria and mammals bifunctional enzymes are employed [Rubio06].